1. Field of Invention
The invention relates to a sewing machine capable of sewing a cloth held by an embroidery frame and, in particular, to a sewing machine having a trace mode where an embroidery area is traced to confirm whether it is within an embroidery frame before sewing.
2. Description of Related Art
Conventionally, an embroidery machine with multiple needles with a single head or multiple heads, comprises an embroidery frame that holds a work cloth in which a pattern is embroidered or a movable frame attaching multiple embroidery frames thereto so as to move in the X- and Y-axis directions on an X-Y coordinate system. The embroidery machine further comprises a needle bar changing mechanism that selectively changes multiple needle bars, and a thread trimming mechanism that allows needle and bobbin threads to be cut, for example, every time a needle bar is changed, for a color change, by the needle bar changing mechanism based on embroidery data. Thus, the embroidery machine enables colorful embroidering with different color threads by moving the embroidery frame and driving these mechanisms.
As to the embroidery frame, a variety of shapes are prepared so as to conform to a size and a shape of a pattern to be embroidered, such as rectangle, that is a basic shape, an oval, ellipse, or circle which are substantially a rectangle with the four corners rounded off in different ways. Using different shape frames, various patterns are embroidered on plain and tubular cloths.
In an embroidery factory, it is usual to perform a trace operation in advance, in order to prevent an accident where the needle hits the embroidery frame during embroidering from happening. The trace operation is to make a master needle bar trace in an embroidery area, by moving the embroidery frame based on embroidery data, in order to check whether the pattern is placed within the embroidery frame.
In Japanese Patent Publication No. 3-67436, the applicant of this application proposed a sewing machine that enables an operator to confirm a sewing area. In the sewing machine, a trace area in a rectangle circumscribing an outline of a pattern is calculated based on the embroidery data, and the embroidery frame is moved along the outline of the tracing area before embroidering. Because the operator watches to check whether the entire range of the trace area fits inside of the embroidery frame, the accident where the needle hits the embroidery frame can be prevented.
However, in the above-mentioned patent, the calculation to determine the trace area is performed for a rectangle only even if the pattern has a shape close to an oval or ellipse, i.e., no pattern extends to the four corners of the rectangle. As a result, the trace area greatly differs from the actual pattern shape, which may cause a trace error because only corners of the entire range of the trace area are partially out of the embroidery frame although the actual pattern itself does not reach the corners. In this case, an embroidery frame considerably larger than the actual pattern size is needed.
When an embroidery frame considerably larger than the actual pattern size is used, appropriate tension can not be applied to a cloth held by the frame, stitches are not formed beautifully, and the entire quality of the embroidery becomes worse in the end.
In Japanese Patent Publication No. 5-61954, a trace operation is performed along an outline that is almost identical to an actual pattern. The outline is determined based on the relationship among all needle points in the embroidery data. Because the outline determined in this manner is used for a trace operation, it is possible to precisely confirm whether the entire of embroidery area is surely set inside the embroidery frame.
However, the calculation to determine the outline in this manner includes many steps because it has to be done based the relationship among all needle points in the embroidery data. For a large pattern to be embroidered, with 20,000 to 50,000 stitches, its data processing volume becomes enormous, and its processing time is consequently prolonged. In addition, its trace operation time is also prolonged since a trace is performed perfectly along all needle points on the outline. This means the operator must observe the trace operation during the same prolonged time, which lowers efficiency in the trace checking operation and finally affects the entirety of embroidering operations.
On the other hand, a conventional trace is performed, based on the embroidery data, at a uniform speed without stopping. However, in a case that the embroidery area is very close to the embroidery frame, it is difficult to determine whether there is a possibility of an accident where the needle bar hits the embroidery frame, only at one try. As a result, the trace should be repeatedly performed until the operator recognizes whether the trace has a problem that leads to such an accident.
As against this, a more accurate and prompt trace method is proposed in Japanese Laid-Open Patent Application Publication No. 9-137365. The trace speed is controlled by a program that enables the trace speed to change based on the positional relationship between the embroidery area and the embroidery frame.
However, the trace operation is observed by the operator. In the above-mentioned trace methods, it is impossible for an operator to change the trace speed because trace operation is controlled by the program. Further, when the operator wants to check a part of the trace again, it is necessary to retrace the embroidery area from the very beginning.